This invention relates to a telephoto type objective comprising a front lens group and a rear lens group which includes at least two members movable for focusing.
It is known to provide a symmetrical lens or triplet lens of which the rearmost lens element is movable for focusing, as disclosed in U.S. Pat. Nos. 2,503,789 and 3,020,804. In these patents, however, no account is taken of the fact that the image quality will be deteriorated when only one lens element is moved for focusing.
It is also known to provide a telephoto lens composed of convergent front and divergent rear lens groups, both of which are made movable as a unit for focusing purposes. In the course of development of new types of telephoto lens, however, it is considered possible to achieve the focusing by moving either the front lens group or the rear lens group.
If the telephoto lens is made bodily movable for focusing, as the amount of axial movement of the entire lens system through the focusing range is far larger than that of axial movement of the ordinary lens, and the weight and bulk of the telephoto lens are relatively large, it is proven that the focusing mechanism such as those employing helicoid structure requires not only large constructional dimensions with high production cost thereof but also a large driving torque with difficult management thereof.
In the case of focusing provision made at the rear lens group, it is possible to minimize the dimensions of the focusing mechanism with improved management thereof. Another advantage of this type telephoto lens is that the front lens group may be supported in fixedly secured relation to a camera body as the physical length between the front lens vertex and the image plane within the camera housing is maintained constant, thereby minimizing the probability of producing a jiggle or oscillation of the image at the focal plane by small accidental motion of the lens-and-camera assembly which is otherwise encountered particularly when the objective lens of the camera is telephoto in nature. With this type of telephoto lens, however, it is made more difficult to prevent deterioration of image quality resulting from variation of aberrations with focusing.
This difficulty can be overcome by employing an additional lens member arranged to be movable along with the focusing member to compensate for variation of aberrations caused by the focusing member. An example of such method is described in U.S. Pat. No. 3,854,797 assigned to the same assignee as the patent application, in which the rear lens group is divided into at least two sub-groups of which the image side one has a negative refracting power and is arranged to be moved toward the rear as the entire lens system is focused for shorter object distances, and another sub-group on the object side which has a strong divergent surface and is arranged to be moved toward the front in the same focusing as above. This arrangement, however, leads to an increase in the diameter of the front sub-group, or otherwise those of the axial rays which are to pass through the marginal portion of the rear sub-group are blocked by the lens cell holding the front sub-group when the entire system is focused for shorter object distances.